I don't really know why, but I answered a guess and got it correct, so If you trust me and what I am saying you will get the correct answer. However, If you don't trust me I completely understand why, but hope you use my answer anyway. Hope this can help you. If it doesn't please forgive me and the website I used.
The easiest way to do this is for you to subtract all the sides from the total perimeter since the perimeter is all of the sides added and your remainder is your answer
